科研之友
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摘要
Ultramafic xenoliths from Tasmanian basalts were analysed for elemental and isotopic compositions of noble gases, for greater understanding of the evolution of the underlying subcontinental lithosphere. The noble gas isotopic compositions indicate contributions of radiogenic and MORB-like components to the xenoliths. The MORE-like component is characterized by (3)He/(4)He ratios between 7 and 9 Ra while the radiogenic component shows lower (3)He/(4)He ratios than those in MORE (8.75 +/- 2.14 Ra, Graham, 2002). The identified radiogenic component in xenoliths from Tasmania may stem from subduction events, during which the mantle wedge and subcontinental lithospheric mantle were metasomatised by U- and Th-rich fluids. As subduction processes have played important role in the evolution of Eastern Australia during the Paleozoic, the radiogenic component is probably associated with this event. Incorporation of MORB-like noble gases in the subcontinental lithospheric mantle beneath Tasmania is likely related to extensional rifting and associated opening of the Tasman Sea in the Late Mesozoic and Cenozoic. The identified noble gas components of the xenoliths are consistent with the geochemical character of the host basalt based on the radiogenic isotope and trace element systematics. This agreement and the petrographic observations (e.g. fluid inclusion trails connected with the reaction rim of the minerals close to the host basalt, spongy texture) indicate that fluids (noble gases, CO(2)) most likely propagated from the host basalts to the xenoliths forming fluid inclusions. The observed noble gas isotopic heterogeneity in mantle-derived xenoliths from Tasmanian to North Queensland reflects isotopic heterogeneity at regional scale in the subcontinental lithospheric mantle beneath eastern Australia.
- 出版日期2010-11-1
